While molecular sieves in bead form are widely used in compressed gas filters for their unique capability of selectively adsorbing a contaminant, such as moisture, from a compressed gas passed therethrough, it has been recognized that the mutual abrasion of the beads produces dust, which, picked up by the gas, can detrimentally affect air brakes or other gas-operated devices when the product gas is so employed. As in Hankison et al. U.S. Pat. No. 3,464,186 and Frantz U.S. Pat. No. 4,097,248, the dust-producing interabrasion of the desiccant particles was somewhat mitigated against by resort to a compressed coil spring for compacting the particles. However, whether the spring was contained in the canister containing the particulate desiccant or mounted in the housing of the filter unit into which the desiccant canister was inserted, the shortcoming of the coil spring was that, since the force exertable by such a spring progressively decreases as the spring expands, it could not adequately compensate for settlement of the particles. A practical solution for the problem posed by the progressive settlement of the particles is disclosed in L. Frantz U.S. Pat. No. 4,029,486 in which a compacting spring is replaced by a pneumatic compactor mounted in the housing of the filter unit and having a piston actuated by compressed gas with which the compactor is automatically chargeable during the filter unit's filter cycle and acting through a floating follower in a canister loaded with a particulate desiccant for maintaining a constant compacting force on the desiccant regardless of settlement of the latter. While manually dischargeable to permit removal of an exhausted desiccant canister and insertion of a newly loaded replacement in the housing of a filter unit, a compactor of the L. Frantz patent could not compact the particulate desiccant prior to insertion of the canister in the housing and had to be discharged whenever a canister was inserted into or removed from the filter unit housing. It is to an improvement on the compactor of the L. Frantz patent possessing these capabilities that the present invention is particularly directed.